The present invention provides a method for determining an optimal operation point with respect to SOC in a hybrid electric vehicle, in which, as a conversion factor (fuel equivalent factor) that can quantitatively compare the amount of fuel consumed by an engine with electrical energy consumed by a motor, a factor for obtaining a desired final SOC with respect to an initial SOC is determined, and an optimal operation state based on a current SOC of the hybrid electric vehicle is determined using the factor, thus improving fuel efficiency of the hybrid electric vehicle.